1. Field of the Invention
The present invention relates to a measuring device, more particularly to a measuring device for measuring insulation resistance of an electric vehicle.
2. Description of the Related Art
Referring to FIG. 1, an electric vehicle 90 usually includes a high voltage system 91 and a low voltage system 92. The high voltage system 91 is primarily configured for driving a motor 911 and thus a high voltage power source VB is adopted for power supply. The low voltage system 92 is primarily configured for providing power for electric circuits generally operating at low voltages. A ground side 921 of the low voltage system 92 is electrically connected to a vehicle body 93 of the electric vehicle 90 so as to form a ground system. Since the high voltage system 91 is not connected to the vehicle body 93, the high voltage system 91 is electrically insulated from the low voltage system 92.
After assembly of the electric vehicle 90, a high potential insulation resistance Ra is a resistance between a high potential side 912 of the high voltage system 91 and the vehicle body 93, and a low potential insulation resistance Rb is a resistance between a low potential side 913 of the high voltage system 91 and the vehicle body 93. The high potential insulation resistance Ra and the low potential insulation resistance Rb are crucial in consideration of operational safety. If the high potential insulation resistance Ra and the low potential insulation resistance Rb are insufficient, electric leakage may occur, and operation of the high voltage system 91 and the low voltage system 92 may become unstable and the systems 91, 92 may be prone to damage.
Therefore, it is necessary to measure the high potential insulation resistance Ra and the low potential insulation resistance Rb for ensuring stability and safety of operation of the high voltage system 91 and the low voltage system 92. A leakage detection circuit for an electric vehicle is disclosed in U.S. Pat. No. 7,592,815. However, four resistors, two switches, and an inverter are required so as to form a detecting unit. Therefore, circuit complexity and manufacturing cost associated with implementation of the aforementioned patent are relatively high. Moreover, only a combined leakage resistance value may be derived from mathematical operations, and the high potential insulation resistance Ra and the low potential insulation resistance Rb may not be independently determined.